Single frequency operation of a semiconductor laser can be achieved by employing a hybrid cavity in combination with an external waveguide that includes a Bragg grating. These devices are referred to as waveguide DBR laser sources. FIG. 1 illustrates one such device referred to as a fiber DBR laser comprising a semiconductor laser chip operated as a gain element with its output optically coupled to an optical fiber that includes a fiber Bragg grating. The semiconductor laser chip has an anti-reflective (AR) coating on its front exit facet and a high reflective (HR) coating on its other surface and operates, therefore, as a gain element. Light emitted via the exit facet is focused into the fiber. The fiber grating functions as a partial reflector of a narrow wavelength band reflecting a peak wavelength back into the gain element. The laser cavity is thus defined by the HR facet of the gain element and the grating of the fiber. Examples of fiber DBR lasers are disclosed in the articles of Olsson et al., IEEE Journal of Quantum Electronics., Vol. 24, pages 143-147, 1988, and also by Morton et al., Applied Physics Letters., Vol. 64, pages 2634-2636, 1994.
A fiber DBR laser produces an exceptional single frequency beam because of the narrow bandwidth selectivity of the grating reflective feedback. Since this wavelength of the resonant cavity can be precisely selected by the fiber grating, the fiber DBR laser has become an attractive option for sensing and wavelength division multiplexing (WDM) applications. The fiber Bragg grating is a periodic grating which written within the core of an optical fiber doped with at least one photorefractive element and, upon exposure to UV light via a grating mask, a grating of periodic different refractive index regions is formed in the core of the fiber. Thus, the fiber Bragg grating is a narrow bandwidth wavelength selective partial reflector for forming the resonant cavity while allowing a larger portion of light to pass through and along the fiber as light output.
A fundamental problem arising from employment of a fiber Bragg grating is its sensitivity to temperature changes. With increasing operational or environmental temperature, the fiber length will expand, extending the grating period as well as increasing the refractive index of the fiber. Consequently, the center wavelength of the reflected bandwidth of the fiber grating will be shifted slightly toward a longer wavelength. For a silica fiber this translates to a shift of approximately 0.1 nm for every temperature increase of 10.degree. C. This can be a significant problem for many applications requiring precise wavelength output control. Temperature changes not only shift the operating peak wavelength but also result in a sufficient shift to cause a longitudinal mode hop, i.e., operational wavelength operation from one longitudinal mode to an adjacent mode.
Although these problems resulting from these temperature dependent factors have not been solved for fiber DBR lasers, some progress has been made toward stabilization of the reflected peak wavelength of a fiber Bragg grating. In particular, Morey et al., U.S. Pat. No. 5,042,898, provides a device for mounting a fiber grating under strain. The mounting device is an arrangement of two materials of greatly differing coefficients of thermal expansion. The fiber is mounted via the device in such a way that, as temperature increases, the strain on the fiber decreases. Thus, the thermal expansion and thermally induced refractive index change of the grating are compensated for by the release in fiber strain. In the reference to G. W. Yoffe et al., "Passive Temperature Compensating Package for Optical Fiber Gratings", Applied Optics, Vol. 34(30), pp. 6859-6861, Oct. 20, 1995, a thermal compensating package is disclosed which holds the Bragg wavelength of the grating relatively fixed over a temperature range of 120.degree. C., i.e., wavelength stabilization of the grating is addressed. However, compensation in Yoffe et al. is reference only with respect to changes occurring in an optical fiber and there is no disclosure of how to prevent longitudinal mode hopping.
Although it is suggested in the prior art that temperature compensated fiber grating packages may be employed in conjunction with external laser cavities, there is no disclosure as to how this may be effectively accomplished. It is far from obvious how one would use such a device or if such a device as shown in the prior art will work in external cavity lasers. A major problem to resolve is that the various components comprising the fiber DBR laser have different temperature dependent parameters and these parameter variations must also be considered along with the variations in the fiber grating parameters so that the laser, as a whole, can function in a stable manner.
It is the object of the present invention to provide a thermal compensator for a waveguide DBR laser that suppresses mode hopping.
It is another object of this invention to provide a temperature-compensating package that compensates for temperature changes in the laser cavity of a waveguide DBR laser including changes occurring in a semiconductor gain element portion of the cavity.
It is a further object of the invention to provide a package which simultaneously compensates for the changes in the lengths and indices of refraction of the optical fiber, gain element, Bragg grating element and the intervening cavity space of a waveguide DBR laser due to operational temperature changes to stabilize wavelength and suppress longitudinal mode hopping.